As shown in FIGS. 1 and 2, a known prior art carburetor has a fuel adjustment assembly 16 with low and high speed adjustable needle valves 17, each threaded into a needle valve receptacle 18 in a carburetor body 19. To permit adjustment of fuel flow, each valve receptacle 18 communicates with a separate fuel passage (not shown) in the carburetor body. Each needle valve 17 generally includes a distal tip 21, an enlarged head 22 and a threaded shank 23 disposed between the tip and the enlarged head. The threaded shank 23 of the needle valve 17 engages a female threaded portion 20 of the needle valve receptacle 18. The tip 21 of the valve 17 may be positioned within an axially-aligned needle seat orifice of the fuel passage and can be axially advanced and retracted, by rotation of the needle valve 17, to adjust the fuel flow rate. Axial advancement and retraction of the distal tip 21 in the seat orifice respectively decreases and increases the amount of fuel that can flow through the orifice by decreasing and increasing the cross-sectional area through which fuel flows. The enlarged head 22 of the needle valve 17 is rotated by using a tool such as a screwdriver inserted into a diametric slot 24 in the head 22 which protrudes from the carburetor body 19. In some such assemblies, to prevent inadvertent or un-commanded rotation of the needle valve 17, an adjustment needle limiter cap 25 is placed over the screw head 22 and is engagable with an adjacent stop.
Because of machining tolerances and limitations during manufacture, fuel adjustment assemblies of this type include enough clearance between the threads of shank 23 of the needle valve 17 and the valve receptacle 18 to allow for lateral and axial movement of the tip 21 relative to the needle seat orifice when force is applied to the valve head 22. This lateral and axial movement can change the size of the orifice flow area enough to result in fuel flow rate changes of up to 20% from an optimum fuel flow rate determined by the manufacturer. Fuel flow rate changes caused by this needle “slop or wobble” result in excessively rich or lean fuel mixtures that undesirably increase exhaust emissions or affect engine performance. Therefore, it is desirable to reduce fuel flow fluctuations through the needle valve 17 and the resulting increase in exhaust emissions and/or deterioration of engine performance by limiting needle slop and wobble.
Not only is it desirable to limit or hold steady the lateral and axial position of the needle valve tip 21 with respect to the orifice and regardless of the valve's rotational position, it is also desirable to maintain the desired setting of the fuel flow in a running engine. Any inadvertent rotation of the needle valve 17, possibly caused by the vibration of a running engine or placement of a conventional limiter cap 25 over the valve's head 22 and after valve adjustments, can alter desired setting of the fuel flow. Therefore it is desirable to restrain the rotation of the needle valve 17 thereby preventing any unintended changes to the fuel flow setting. To do so, traditionally, compression springs 26 are disposed concentrically about the shank 23 and axially between the carburetor body 19 and the head 22 of the needle valve 17. The spring-induced axial force produces increased frictional forces amongst the threads between the carburetor body 19 and the needle valve 17, thus resisting needle valve rotation and alteration of the fuel flow setting. Unfortunately, springs 26 are relatively expensive to manufacture, and to produce sufficient frictional forces must be relatively long, causing the needle valves 17 to project a substantial distance outward from an otherwise compact carburetor.
One example of a stabilizing system for a fuel adjustment assembly is disclosed in U.S. Pat. No. 6,540,212, issued Apr. 1, 2003, assigned to Walbro Corporation, and incorporated herein by reference. This U.S. patent generally describes the carburetor fuel adjustment assembly 16 illustrated in FIGS. 1 and 2, having both the spring 26, as described above, and a retainer or clip 27 which exerts a lateral force upon the spring 26 and indirectly upon the needle valve shank 23 to produce further friction and minimize unintentional valve rotation. Unfortunately, the retainer clip 27 is external to the carburetor body 19 and thus subject to possible damage.
Another example of a stabilizing system for a fuel adjustment assembly is disclosed in U.S. Pat. No. 5,948,325, issued Sep. 7, 1999, assigned to U.S.A. Zama, Incorporated, and incorporated herein by reference. In this U.S. patent, a resilient fastening member is press fitted into a pre-drilled bore of a carburetor body. Once located in the bore, two needle valve receptacle cavities are bored transversely into the body and completely through the fastening member, thus the fastening member has a diameter which is larger than the receptacle cavity. Threads are then formed in the receptacles by rolling threads into both the carburetor body and the fastening member, for threadable receipt of the needle valves. Because the fastening member is resilient, it does not undergo plastic deformation during thread rolling and does not form actual female threads as does the metal portion of the receptacle or carburetor body. When the needle valves are installed and adjusted, the fastening member produces a frictional force upon the male threaded valve shanks which assuredly holds the valves in their adjusted position. Unfortunately, manufacture of the fastening member is expensive because after installation into the carburetor body, it must be drilled to produce two axially spaced through-holes and the threads must be formed by the thread rolling. Moreover, the process of manufacturing the carburetor is restricted because both the receptacles and the fastening member must be machined simultaneously. Yet further, the bore required to receive the fastening member is relatively long because the through-holes, and thus the receptacles, are spaced radically away from one-another and with respect to the longitude of the bore. This requires a large portion of the carburetor body to be dedicated for the bore and fastening members, and which might otherwise be utilized for other carburetor features, producing a relatively larger and less compact carburetor.